How to ad Images to featured boxes

Hi!

I have following problem. On my landing page i use featured boxes who should display my image from my post. but this isnt happen. I only see a blank pic and the text from my post. In another post its described that the featured boxes automatically use the image when these are assigned to the source posts.

how do i assign the image to the source post!? so i it will apear in the box.

THX for your help!